Ultrasonic energy is known to be used for welding foil layers of a packaging product such as a bag, for example. To weld the plastic parts of the bag, the material is caused to vibrate under pressure at a high frequency (20 kHz-1 Ghz). As a result, the material structure is deformed so that heat is generated, which heat causes the foil layers to melt. The heat is generated internally in the foil layers to be welded as a result of the friction generated by the vibration in the foil layers.
EP 1.535.723, for example, describes a known device comprising, for example, a sonotrode as well as an anvil, which device is used for ultrasonic welding of foil layers for forming bags.
With the known device, of which a section is diagrammatically shown in the appended FIG. 1, the flow-out section U which ends where at least one of the foil layers to be welded is no longer supported by the anvil, is relatively short compared to the welding section L, which is caused by the shape of the anvil. In operation the liquid plastic generated by means of ultrasonic energy is pressed outwards when the foil layers are welded in the welding section L where the foil layers are pressed together. Due to the shape of the flow-out section the liquid plastic is pressed outwards in an uncontrolled manner, where it cools down and forms a welded joint which is represented in the appended FIG. 1. Depending on the shape accidentally obtained from the liquid plastic it is possible for defective welded joints to be formed side by side with good welded joints as a result of the uncontrolled flowing out of the liquid plastic. The defective welded joints may have weak spots at the beginning and end where the liquid plastic has been able to accumulate in an uncontrolled manner and these weak spots are the result of poor melt fusions which, when loaded, may result in fractures and cracks. These fractures and cracks may be the cause of a part of or even the entire welded joint being ripped open, causing the contents of the bag to be exposed to the atmosphere, which may detrimentally affect the keeping qualities of the products, for example, or may lead to leakages in the case of packaged liquids.
In DE 101.47.565 a device is described for sealing the housings of galvanised elements. The known device comprises a sonotrode and an anvil of which at least one is provided with a raised profile and is surrounded by a planar surface so that ridges can no longer develop on the outside contour of the housings of the galvanic elements. DE 101.47.565 is considered to be the most pertinent prior art of record.